1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to an in-plane switching mode liquid crystal display device formed such that the black matrix on an upper substrate is included in the non-aperture region on a lower substrate.
2. Discussion of the Related Art
Generally, a liquid crystal display (LCD) device operates by the optical anisotropy and polarization of a liquid crystal material therein. Since the liquid crystal material includes liquid crystal molecules, each having a thin and long structure, the liquid crystal material has a specific orientation according to the alignment direction of the liquid crystal molecules. Hence, the alignment direction of the liquid crystal molecules can be controlled by applying an external electric field to the liquid crystal.
As the alignment of the liquid crystal molecules are changed by applying an electric field, light polarization caused by the optical anisotropy of the liquid crystal material is modulated to display image information.
Liquid crystal material can be classified into positive (+) liquid crystal having a positive dielectric anisotropy and negative (−) liquid crystal having a negative dielectric anisotropy depending on its electrical properties. Liquid crystal molecules having a positive dielectric anisotropy are arranged such that their long axes are parallel with the direction of an applied electric field, and liquid crystal molecules having a negative dielectric anisotropy are arranged such that their long axes are normal to the direction of an applied electric field.
Nowadays, an active matrix LCD in which thin film transistors and pixel electrodes connected to the thin film transistors are arranged in a matrix configuration are widely being used because of their high resolution and superior moving picture reproducing capability.
FIG. 1 is a partial exploded perspective view of a general liquid crystal panel.
Referring to FIG. 1, a structure of the liquid crystal panel that is a main element of the LCD device will be reviewed hereinafter.
The LCD device includes an upper substrate 5 and a lower substrate 22. The upper substrate 5 includes a color filter layer 7 having sub-color filters 8 of red (R), green (G), and blue (B) and a black matrix 6 between the sub-color filters 8, and a transparent common electrode 18 formed on the color filter 7. The lower substrate 22 includes pixel regions (P), pixel electrodes 17 formed on the pixel regions (P), and an array of interconnection lines including switching elements (T). A liquid crystal layer 14 as described above is interposed between the upper substrate 5 and the lower substrate 22.
The lower substrate 22 is called an “array substrate”. On the lower substrate 22, a plurality of thin film transistors that function as switching elements are arranged in a matrix shape, near the crossing of gate lines 13 and data lines 15.
The pixel regions (P) are defined by the gate lines 13 and the data lines 15 crossed with the gate lines 13. The pixel electrode 17 formed on the pixel region (P) is made of a transparent conductive material, such as indium-tin-oxide (ITO), having superior light transmittance.
Explaining the operation of the liquid crystal panel, the LCD device constructed as above displays images when liquid crystal molecules of the liquid crystal layer 14 between two substrates are aligned by a voltage applied between the common electrode 18 of the upper substrate 5 and the pixel electrode 17 on the lower substrate 22 to control the amount of light passing through the liquid crystal layer 14.
Described as above, the LCD device structured by the common electrodes 18 over the pixel electrodes 17 operates to align the liquid crystal material by the electric field applied between the top and bottom of the device. The properties of permeability and aperture ratio or the like become superior, and the common electrodes of the upper substrate function as ground connections to prevent the destruction of the liquid crystal cells.
However, the operation of the liquid crystal material driven by the electric field applied to the top and bottom of the device has a disadvantage that its visual angle property (viewing angle) is inferior, and therefore, there has been introduced a new technology to solve the above disadvantage. Hereinafter, an effort to solve the disadvantage of the inferior visual angle property (viewing angle) by using an in-plane switching mode (IPS) LCD device will be reviewed.
FIG. 2 is a sectional view illustrating a part of a related art in-plane switching mode LCD device.
Referring to FIG. 2, the related art in-plane switching mode LCD device includes a pixel electrode 17 and a common electrode 18 on a lower substrate 22. A liquid crystal layer 14 ioperates according to the in-plane electric field formed between the pixel electrode 17 and the common electrode 18 on the lower substrate 22.
Further, an upper substrate 5 is formed in the upper region over the liquid crystal layer 14. The in-plane switching mode LCD device structured as above has a characteristics in that an in- plane electric field 35 is used since the pixel electrode 17 and the common electrode 18 are all formed on the same plane substrate.
Further, an overcoat layer 44 is formed on the upper substrate 5, for protecting each sub-color filters 8. A sealant 40 is formed along the edge of the upper substrate 5 and the lower substrate 22 for adhering the upper substrate 5 and the lower substrate 22.
Further, a black matrix 6 is formed on the upper substrate 5, and since the in-plane switching mode LCD device operates the liquid crystal by a plane electric field, the black matrix 6 is comprised of an organic material (resin) not metal, for preventing the distortion of the electric field.
The black matrix 6 exists in the area between the sub color filters 8 and functions as a light shutter. An aperture region of the liquid crystal panel is defined by the black matrix 6.
The substrates structured as above can be formed by a deposition process, a photolithography process (hereinafter referred to as a ‘photo-process’), and an etching process, etc.
The photo-process utilizes a principle that, when photoresist (hereinafter referred to as a ‘PR’) is exposed to light, a chemical reaction occurs to change the property of the PR. In the photo-process, light is selectively irradiated onto the PR through a mask of a desired pattern, thereby forming the same pattern as the pattern of the mask. The photo-process includes a PR coating step in which a PR corresponding to a general picture film is coated on, an exposure step in which light is selectively irradiated onto the PR using a mask, and a developing step in which the exposed portion of the PR is removed to form a pattern.
Nowadays, the size of the substrate becomes greater than ever with the mass production of large-sized LCD devices.
Therefore, one exposure mask is not enough when forming the black matrix 6 on the upper substrate 5 for large-size LCD devices. That is, as the screen size of the upper substrate 6 is greater than the exposure mask used in the photo-process, during the exposure step, the screen of the upper substrate is divided into a plurality of shots and is repeatedly exposed, which is a step exposure method.
However, while forming the black matrix 6 by the step exposure as above, limitations in the preciseness of the exposure equipment causes reduced picture quality of the LCD, such as the generation of stitch spots, by the stitch failure that the misalignment between the shots occurs, that is, the change of critical division (CD) on the interfacial boundaries between the shots.
As an aperture region for each pixel in the liquid crystal panel is defined by the black matrix 6, the misalignment of the black matrix 6 brings the size difference of each aperture region each sub-pixel 10, and the difference of the brightness in each sub-pixel 10, which causes the stitch failure as above.
Therefore, in the above structure of the related art LCD devices, the amount of light penetrating each sub-pixel 10 is different depending on the shapes of the black matrix 6, which determines the aperture region of each sub-pixel 10, by the stitch failure, thereby causing spots on display images and reduced picture quality of an LCD device.
Therefore, in the above structure of the conventional LCD devices, the amount of light penetrating each sub-pixel 10 is different depending on the shapes of the black matrix 6, which determines the aperture region of each sub-pixel 10, by the stitch failure, thereby causing spots on display images and reduced picture quality of an LCD device.
FIG. 3 is a plane view partially illustrating a lower substrate of the related art in-plane switching mode LCD device, and FIG. 4 is a plane view illustrating the region including a black matrix of an upper substrate facing a sub-pixel in FIG. 3, and FIG. 5 is an exploded perspective view illustrating a part of the related art in-plane switching mode LCD device.
Referring to FIG. 3, the lower substrate is configured to have a plurality of gate lines 13 and the common lines 54 formed thereon in parallel in the horizontal direction, and in the vertical direction, a plurality of data lines 15 formed thereon crossed with the gate lines 13 and the common lines 54.
As described above, a sub-pixel 10 is defined as a region surrounded by the gate line 13, the common line 54, and the data lines 15, 15′ on the lower substrate.
Further, a gate electrode 31 is formed in one side of the gate line 13, and a source electrode 33 is formed in one side of the data line 15 adjacent to the gate electrode 31 and partially overlaps with the gate electrode 31. A drain electrode 35 is formed to face the source electrode 33 distanced therefrom by a gap, and therefore, all above elements form a thin film transistor region (T).
Further, the common line 54 has a plurality of common electrodes 54a extending from the common line 54. The drain electrode 35 is connected to a lead interconnection line 37 from which a plurality of pixel electrodes 37a extend.
The common electrodes 54a and the pixel electrodes 37a are formed in an alternating manner and are substantially parallel to each other. An image display region of the sub-pixel 10 is formed by the plurality of the common electrodes 54a and the pixel electrodes 37a. 
Further, a storage capacitor 50 is formed in the region where the common line 54 and the plurality of pixel electrodes 37a partially overlap with the common line 54.
The common electrodes 54a formed in the sub-pixel 10 described are held at a common voltage that is received from the common lines 54. Image signals of varying voltage levels are applied to the pixel electrodes 37a from the data line 15 in accordance with a gate voltage supplied via the gate electrode 31.
Therefore, a plane electric field is formed by the voltage applied on the pixel electrodes 37a and the common electrodes 54a, and the alignment degree of the liquid crystal molecules can be varied depending on the intensity of such an electric field to display images.
A block 39 refers to a region displaying images according to the plane electric field between the pixel electrodes 37a and the common electrodes 54a. Each sub-pixel 10 includes a plurality of the blocks 39. Normally, as illustrated in FIG. 3, a four-block type, in which four blocks 39 exist in one sub-pixel 10, is widely used.
The sub-pixels 10 are distributed on the lower substrate in a matrix shape, and when a light (e.g., from a backlight unit (not shown)) passes through a plurality of blocks 39 in each sub-pixel 10, images can be displayed. The area of the plurality of blocks passing the light is an aperture region 41, and on the other hand, the region of the sub-pixel 10 other than where the plurality of blocks 39 are a non-aperture region 42, which does not display images.
However, in the related art, the aperture region formed by the plurality of blocks determines its size by the black matrix formed on the color filter. This will be explained in more detail with reference to FIG. 4.
Referring to FIG. 4, the light passing through the plurality of blocks 39 in the sub-pixel 10 from the backlight penetrates a plurality of sub-color filters (R, G, B) 8 formed on the upper substrate 5 to display various types of images, including a still image or moving images. In the region where images are not displayed, a black matrix 6 is formed to prevent a light leakage phenomenon.
The black matrix 6, in the related art, is formed by including a part of the aperture region, that is, the region of the blocks 39 as well as the thin film transistor region (T) and the data/gate line 13/15. The black matrix partially overlaps with the region of the blocks 39 taking into account the bonding margins of the lower substrate and the upper substrate, or the like.
Accordingly, in the related art LCD device, the aperture ratio is determined by the black matrix 6, and not all the light passing through the aperture region formed on the lower substrate contributes to the display.
Further, referring to FIG. 5, the related art in-plane switching mode LCD panel includes a upper substrate 5 having a black matrix (BM) 6 and sub-color filters (R, G, B) 8, and a lower substrate 22 having the sub-pixels 10 in FIG. 3 aligned in a matrix shape. A liquid crystal material (not shown) fills the gap between the upper substrate 5 and the lower substrate 22.
The images displayed through the plurality of blocks 39 formed in each sub-pixel 10 on the lower substrate 22 are bounded by the black matrix 6 formed on the upper substrate 5.
That is, the black matrix 6 reaches even the peripheral region of the aperture region 41 as well as the region including the non-aperture region 42 of the lower substrate 22, and accordingly, the displayed region in the related art in-plane switching mode LCD device is defined by the black matrix 6.
However, as described above, with the enlargement of the lower substrate 22 and the upper substrate 5 in recent years, the black matrix 6 cannot be formed using just one exposure mask. That is, as the display size of the upper substrate 5 is greater than that of the exposure mask used in the photo-process. During the exposure step, the screen of the upper substrate is divided into a plurality of shots and is repeatedly exposed, which is a step exposure method.
However, while forming the black matrix 6 by the step exposure as above, limitations in the preciseness of the exposure equipment causes a stitch failure due to the misalignment between the shots. Thus, the light penetration region, that is, the image displayed region, is not the same in every sub-pixel 10, illustrated in FIG. 4, as the black matrices 6 in the sub-pixel 10 are misaligned so as not to coincide with each other exactly. Therefore, brightness varies in each sub-pixel 10, and the stitch spot failure occurs on the image of the LCD device.